1. Technical Field
The present invention related to a driving device for a liquid crystal panel of an active matrix driving system according to thin film transistor (hereinafter referred to as TFT) driving or the like and an image display apparatus including the driving device.
2. Related Art
For example, an electronic apparatus disclosed in JP-A-11-282426 is known as the image display apparatus using the liquid crystal panel of the active matrix driving system according to TFT driving. FIG. 7 is a schematic diagram of a liquid crystal panel 202 provided in the electronic apparatus disclosed in JP-A-11-282426 and a driving device 203 for the liquid crystal panel.
In the liquid crystal panel 201, a large number of scan lines Y1 to Ym and data lines X1 and Xn, which are arranged vertically and horizontally, respectively, and a large number of pixel electrodes 240 corresponding to respective intersections of the scan lines and the data lines are provided on a glass substrate. In addition to the scan lines, the data lines, and the pixel electrodes 240, peripheral circuits such as a scan line driver 230, a data line driver 220, sampling circuits SH1 to SHn, and pixel TFT circuits ST1 to STn are provided on the glass substrate. Moreover, liquid crystal cells corresponding to the respective pixel electrodes are filled between two opposed glass substrates to form the liquid crystal panel 201.
The driving device 203 is a signal generating circuit including a frequency dividing circuit or the like. An operation clock CLK and a horizontal synchronizing signal HSYNC and a vertical synchronizing signal VSYNC of an image signal are supplied to the driving device 203. The driving device 203 generates a start signal DX, a clock signal CLX, an inverted clock signal CLXN, an enable signal ENBX, and the like serving as timing signals on the basis of the operation clock CLK with the horizontal synchronizing signal HSYNC as a trigger.
The data line driver 220 including selection circuits L1 to Ln generates sampling signals S1 to Sn for determining driving timing for the sampling circuits SH1 to SHn on the basis of plural timing signals supplied from the driving device 203.
The sampling circuit SH1 to SHn including switching elements such as TFTs outputs image signals VID1 to VID6, which are expanded into six phases, to the pixel TFT circuits ST1 to STn only for a period in which the sampling signals S1 to Sn are at a high level.
Scan signals Y1 to Yn outputted from the scan line driver 230 are inputted to the pixel TFT circuits ST1 to STn. The pixel TFT circuits ST1 to STn outputs the image signals VID1 to VID6 to the pixel electrodes 240 only for a period in which the scan signals Y1 to Yn are at a high level. In this way, a video represented by the image signals VID1 to VID6 is displayed on the liquid crystal panel 201.
In the liquid crystal panel 201, when characteristics of a shift register included in the data line driver 220 and transistors constituting NAND circuits and the like of the selection circuits L1 to Ln are deteriorated, overlap occurs among the sampling signals S1 to Sn and a ghost image may be displayed.
In view of such a problem, the electronic apparatus disclosed in JP-A-11-282426 adjusts the period in which the sampling signals S1 to Sn are at a high level according to the enable signal ENBX to eliminate the overlap of the sampling signals S1 to Sn and prevent a ghost from being caused.
The ghost image may also be caused because of shift between a period in which the image signals VID1 to VID6 reach a saturated level and the period in which the sampling signals S1 to Sn are at a high level.
The image signals VID1 to VID6 are integrated by an internal circuit of the liquid crystal panel 201, whereby an edge of a waveform thereof is blunted. Therefore, if the period in which the image signals VID1 to VID6 reach a saturated level and the period in which the sampling signals S1 to Sn are at a high level do not coincide with each other, a ghost image is displayed.
The period in which the sampling signals S1 to Sn are at a high level may temporally shift from the period in which the image signals VID1 to VID6 reach a saturated level because characteristics of circuit elements and the like constituting data line driver 220 and the sampling circuits SH1 to SHn change as a result of a temperature change and aged deterioration at the time when the liquid crystal panel 201 is used.
In the electronic apparatus disclosed in JP-A-11-282426, it is possible to prevent a ghost image due to overlap of the sampling signals S1 to Sn from being caused. However, JP-A-11-282426 does not take into account a ghost image caused by shift between the period in which the image signals VID1 to VID6 reach a saturated level and the period in which the sampling signals S1 to Sn are at high level. The latter period changes as a result of a temperature change and aged deterioration at the time when the liquid crystal panel 201 is used.
A ghost image caused by temporal shift between a period in which images signals reach a saturated level and a period in which sampling signals are at a high level will be explained.
FIG. 6A is a diagram showing an appropriate image in which a ghost image is not caused and states of image signals representing the image and sampling signals.
A black substantially square window pattern 301 is displayed with a light gray background in an image 300 represented by an image signal VID. The image signal VID is expanded into six phases and supplied to the liquid crystal panel 201 as the image signals VID1 to VID6.
The image signals VID1 to VID6 are represented by a waveform having a voltage level (3V) indicating light gray and a voltage level (2V) indicating black. The image signals VID1 to VID6 are integrated by the internal circuit of the liquid crystal panel 201, whereby the edge of the waveform thereof is blunted. Thus, the image signals VID1 to VID6 need to be outputted to the pixel TFT circuits ST1 to STn in the period in which the image signals VID1 to VID6 reach a saturated level (e.g., a period as late as possible in image signal periods Ta and Tb).
A period Qa in which a sampling signal Sk is at a high level (a high-level period Qa of a sampling signal Sk) determines timing for inputting the image signals VID1 to VID6 to pixel TFT circuits corresponding to pixels P1 to P6 on a left side of the window pattern 301.
The high-level period Qa temporally coincides with a period in which the image signals VID1 to VID6 reach a saturated level (3V) of light gray in the image signal period Ta. The image signals VID1 to VID6 representing light gray are inputted to respective pixel electrodes of the pixels P1 to P6.
A period Qb in which a sampling signal Sk+1 is at a high level (a high-level period Qb of a sampling signal Sk+1) determines timing for inputting the image signals VID1 to VID6 to pixel TFT circuits corresponding to pixels P7 to P12 in the window pattern 301.
The high-level period Qb temporally coincides with a period in which the image signals VID1 to VID6 reach a saturated level (2V) of black in the image signal period Tb. The image signals VID1 to VID6 representing black are inputted to respective pixel electrodes of the pixels P7 to P12.
Thus, in a state shown in FIG. 6A, a ghost is not caused at the left end of the window pattern 201.
A line of the pixels P1 to P12 has been explained as an example. However, images are displayed at the same timing not only on the line but also on all lines on the liquid crystal panel 201. Thus, a ghost is not caused in the image 300 as a whole.
FIG. 6B is a diagram showing an image in which a ghost is caused because sampling signals are temporally ahead of image signals and states of the image signals representing the image and the sampling signals.
In FIG. 6B, the sampling signals Sk and Sk+1 temporally advance because of influences of a temperature change and aged deterioration of the liquid crystal panel 201. Thus, a part of the high-level period Qb shifts from the saturated level (2V) of black in the image signal period Tb in the image signals VID1 to VID6 and temporally overlaps a voltage level close to light gray.
Therefore, a part of the image signals VID1 to VID6 of the voltage level close to light gray are inputted to the respective pixel electrodes of the pixels P7 to P12 other than the image signals VID1 to VID6 that reach the saturated level (2V) of black. As a result, the image signals are mixed to cause a ghost of dark gray A on an inner side of the left side of the window pattern 301.
At this point, the same phenomenon occurs in continuous six pixels on an outer side of a right side of the window pattern 301. A part of the image signals VID1 to VID6 of the voltage level close to light gray are inputted to respective pixel electrodes on the right side other than the image signals VID1 to VID6 that reach the saturated level (2V) of black. As a result, the image signals are mixed to cause a ghost of dark gray B on the outer side of the right side of the window pattern 301 as well.
Moreover, the ghost is caused not only on the line of the pixels P6 to P12 but also on all the lines on the liquid crystal panel. Thus, a ghost of the dark gray A is caused on the inner side of the left side of the window pattern 301 and a ghost of dark gray B is caused on the outer side of the right side of the window pattern 301. Color strengths of the thick gray A and the thick gray B vary depending on a degree of temporal advance of the sampling signals Sk and Sk+1.
FIG. 6C is a diagram showing an image in which a ghost is caused because sampling signals are temporally delayed behind image signals and states of the image signals representing the image and the sampling signals.
In FIG. 6C, the sampling signals Sk and S+1 are temporally delayed behind the image signals VID1 to VID6 because of influences of a temperature change and aged deterioration of the liquid crystal panel 201. Thus, a part of the high-level period Qa shifts from the saturated level (3V) of light gray in the image signal period Ta in the image signals VID1 to VID6 and temporally overlaps a voltage level close to black.
Therefore, a part of the image signals VID1 to VID6 of the voltage level close to black are inputted to the respective pixel electrodes of the pixels P1 to P6 other than the image signals VID1 to VID6 that reach the saturated level (3V) of light gray. As a result, the image signals are mixed to cause a ghost of dark gray C on the outer side of the left side of the window pattern 301.
At this point, the same phenomenon occurs in continuous six pixels on an inner side of the right side of the window pattern 301. A part of the image signals VID1 to VID6 of the voltage level close to black are inputted to the respective pixel electrodes on the right side other than the image signals VID1 to VID6 that reach the saturated level (3V) of light gray. As a result, the image signals are mixed to cause a ghost of dark gray D on the inner side of the left side of the window pattern 301.
Moreover, the ghost is caused not only on the line of the pixels P6 to P12 but also on all the lines on the liquid crystal panel. Thus, a ghost of the dark gray C is caused on the inner side of the left side of the window pattern 301 and a ghost of dark gray D is caused on the outer side of the right side of the window pattern 301. Color strengths of the thick gray C and the thick gray D vary depending on a degree of temporal advance of the sampling signals Sk and Sk+1.
In the above explanation, the liquid crystal panel 201 is applied to monochrome display. However, the phenomenon described above occurs even if the liquid crystal panel 201 is applied to color display.
For example, when the liquid crystal panel 201 is a liquid crystal panel applied to color display that colors transmitted light using a color filter of R (red), G (green), or B (blue) for each of the pixels, one color is formed by three continuous pixels. Thus, the three continuous pixels are equivalent to one pixel of the liquid crystal panel applied to monochrome display.
As described above, the electronic apparatus in the past has a problem in that it is difficult to completely prevent a ghost due to temporal shift of timing signals caused by a temperature change and aged deterioration at the time when the liquid crystal panel is used.